Dielectric resonators are used in many circuits for concentrating electric fields. For instance, they are commonly used as filters in high frequency wireless communication systems, such as satellite and cellular communication applications. They can be used to form oscillators, triplexers and other circuits, in addition to filters. Combline filters are another well known type of circuit used in front-end transmit/receive filters and diplexers of communication systems such as Personal Communication System (PCS), and Global System for Mobile communications (GSM). The combline filters are configured to pass only certain frequency bands of electromagnetic waves as needed by the communication systems.
FIG. 1 is a perspective view of a typical dielectric resonator of conventional design. As can be seen, the resonator 10 is formed as a cylinder 12 of dielectric material with a circular, longitudinal through hole 14. FIG. 2 is a perspective view of a microwave dielectric resonator filter 20 of the prior art employing a plurality of dielectric resonators 10a, 10b, 10c, 10d. The resonators 10a, 10b, 10c, 10d are arranged in the cavity 22 of a conductive enclosure 24. The conductive enclosure 24 typically is rectangular. The enclosure 24 commonly is formed of aluminum and is silver-plated, but other materials also are well known. The resonators 10a, 10b, 10c, 10d may be attached to the floor 44 of the enclosure, such as by an adhesive, but also may be suspended above the floor of the enclosure by a low-loss dielectric support, such as a post or rod.
Microwave energy is introduced into the cavity by an input coupler 28 coupled to an input energy source through a conductive medium, such as a coaxial cable. Signals also may be coupled into (and out of) a dielectric resonator circuit by other techniques, such as microstrips positioned on the bottom surface of the enclosure 24 adjacent the resonators.
That energy is electromagnetically coupled between the input coupler and the first dielectric resonator. Coupling may be electric, magnetic, or both. Conductive separating walls 32a, 32b, 32c, 32d separate the resonators from each other and block (partially or wholly) coupling between physically adjacent resonators 10a, 10b, 10c, 10d. Particularly, irises 30a, 30b, 30c in walls 32 control the coupling between adjacent resonators 10a, 10b, 10c, 10d. Walls without irises generally prevent any coupling between adjacent resonators separated by those walls. Walls with irises allow some coupling between adjacent resonators separated by those walls. By way of example, the dielectric resonators 10a, 10b, 10c, 10d in FIG. 2 electromagnetically couple to each other sequentially, i.e., the energy from input coupler 28 couples into resonator 10a, resonator 10a electromagnetically couples with the sequentially next resonator 10b through iris 30a, resonator 10b couples with the sequentially next resonator 10c through iris 30b, and so on until the energy is coupled to the sequentially last resonator 10d. An output coupler 40 is positioned adjacent the last resonator 10d to couple the microwave energy out of the filter 20. Wall 32a, which does not have an iris, prevents the field of resonator 10a from coupling with physically adjacent, but not sequentially adjacent, resonator 10d on the other side of the wall 32a. 
Generally, both the bandwidth and the center frequency of the filter must be set very precisely. Bandwidth is dictated by the magnitude of coupling between the dielectric resonators and, therefore, is primarily a function of (a) the spacing between the individual dielectric resonators 10a, 10b, 10c, 10d of the circuit and (b) the metal between the dielectric resonators (i.e., the size and shape of the housing 24, the walls 32a, 32b, 32c, 32d and the irises 30a, 30b, 30c in those walls, as well as any tuning screws placed between the dielectric resonators as discussed below). The coupling between adjacent resonators is directly proportional to the distance between them.
The center frequency of a dielectric resonator circuit, on the other hand, is primarily a function of the characteristics of the individual dielectric resonators themselves, such as the dielectric constants of the resonators, the size of the individual dielectric resonators, and the metal adjacent the individual resonators (i.e., the housing and the tuning plates 42 discussed immediately below).
Initial frequency and bandwidth tuning of these circuits is done by selecting a particular size for the resonators, a particular size and shape for the housing (including selection of the separating walls and irises), and a particular spacing between the individual resonators. This is a very difficult process that is largely performed by trial and error. Accordingly, it can be extremely laborious and costly. Particularly, each iteration of the trial and error process requires that the filter circuit be returned to a machine shop for re-machining of the cavity, irises, and/or tuning elements (e.g., tuning plates and tuning screws) to new dimensions. In addition, the tuning process involves very small and/or precise adjustments in the sizes and shapes of the resonators, housing, irises, tuning plates, tuning screws, and cavity. Thus, the machining process itself is expensive and error-prone.
Furthermore, generally, a different housing design must be developed and manufactured for every circuit having a different frequency. Once the housing and initial design of the circuit is established, then it is often necessary or desirable to provide the capability to perform fine tuning of the frequency.
In order to permit fine tuning of the center frequency of such circuits after the basic design is developed, one or more metal tuning plates 42 may be attached to a top cover plate (the top cover plate is see-through in FIG. 2 in order not to obfuscate the invention) generally coaxially with a corresponding resonator 10a, 10b, 10c, 10d to affect the field of the resonator (and particularly the parasitic capacitance experienced by the resonator) in order to help set the center frequency of the filter. Particularly, plate 42 may be mounted on a screw 43 passing through a threaded hole in the top cover plate (not seen in FIG. 2) of enclosure 24. The screw may be rotated to vary the distance between the plate 42 and the respective resonator 10a, 10b, 10c, or 10d to adjust the center frequency of the resonator.
This is a purely mechanical process that also tends to be performed by trial and error, i.e., by moving the tuning plates and then measuring the frequency of the circuit. This process also can be extremely laborious since each individual dielectric resonator and accompanying tuning plate must be individually adjusted and the resulting response measured.
Mechanisms also often are provided to fine tune the bandwidth of a dielectric resonator circuit after the basic design has been selected. Such mechanisms often comprise tuning screws positioned in the irises between the adjacent resonators to affect the coupling between the resonators. The tuning screws can be rotated within threaded holes in the housing to increase or decrease the amount of conductor (e.g., metal) between adjacent resonators in order to affect the capacitance between the two adjacent resonators and, therefore, the coupling therebetween. In fact, it generally is a design goal to space the resonators far enough away from each other that there is no direct coupling between electrically adjacent resonators, but only through the iris walls and tuning screws.
The walls within which the irises are formed, the tuning plates, the tuning screws, and even the cavity all create losses in the system, thereby decreasing the quality factor, Q, of the system and increasing the insertion loss of the system. Q essentially is an efficiency rating of the system and, more particularly, is the ratio of stored energy to lost energy in the system. The portions of the fields generated by the dielectric resonators that exist outside of the dielectric resonators touch all of the conductive components of the system, such as the enclosure 20, tuning plates 42, internal walls 32a, 32b, 32c, 32d, and any tuning screws (not shown in FIG. 1) and inherently generate currents in those conductive elements. Field singularities exist at any sharp corners or edges of conductive components that exist in the electromagnetic fields of the filter. Any such singularities increase the insertion loss of the system, i.e., reduce the Q of the system. Thus, although the iris walls, tuning screws, and tuning plates serve an important function, they are the cause of loss of energy within the system.
Another disadvantage of the use of tuning screws within the irises is that such a technique does not permit significant changes in coupling strength between the dielectric resonators. Tuning screws typically provide tunability of not much more than 1 or 2 percent change in bandwidth in a typical communication application, where the bandwidth of the signal is commonly about 1 percent of the carrier frequency. For example, it is not uncommon in a wireless communication system to have a 20 MHz bandwidth signal carried on a 2000 MHz carrier. It would be very difficult using tuning screws to adjust the bandwidth of the signal to much greater than 21 or 22 MHz.
As is well known in the art, dielectric resonators and dielectric resonator filters have multiple modes of electrical fields and magnetic fields concentrated at different center frequencies. A mode is a field configuration corresponding to a resonant frequency of the system as determined by Maxwell's equations. In a dielectric resonator, the fundamental resonant mode frequency, i.e., the lowest frequency, is normally the transverse electric field mode, TE01 (or TE hereinafter). Typically, the fundamental TE mode is the desired mode of the circuit or system in which the resonator is incorporated. The second-lowest-frequency mode typically is the hybrid mode, H11 (or H11 hereinafter). The H11 mode is excited from the dielectric resonator, but a considerable amount of electric field lies outside of the resonator and, therefore, is strongly affected by the cavity. The H11 mode is the result of an interaction of the dielectric resonator and the cavity within which it is positioned (i.e., the enclosure) and has two polarizations. The H11 mode field is orthogonal to the TE mode field. Some dielectric resonator circuits are designed so that the H11 mode is the fundamental mode. For instance, in dual mode filters, in which there are two signals at different frequencies, it is known to utilize the two polarizations of the H11 mode for the two signals.
There are additional higher order modes, including the TM01 mode, but they are rarely, if ever, used and essentially constitute interference. Typically, all of the modes other than the TE mode (or H11 mode in filters that utilize that mode) are undesired and constitute interference.